Cannot copy paste text with images from some specific websites

This is the expected behavior. Images can't be loaded from a different domain unless the target domain they are loaded from allows it, by configuring CORS for instance. You can reproduce this easily by editing a wiki page with the Wiki editor on your local XWiki instance and using content like this:

[[image:https://www.xwiki.org/xwiki/bin/download/Documentation/AdminGuide/Group%20Management/CreateGroup.png?rev=1.3]]